The demand for a wide variety of colors in polyurethane slabstock foam has resulted in a significant move to blend-on-fly color dosing units based on the use of polymeric colorants. In this case color metering equipment is used to accurately dose two or more colors that are injected into the polyol stream and subsequently mixed in the foam mixhead to provide the correct shade and depth of color. The biggest advantage of this type of approach is that now an unlimited number of colors can be made from 4 or 5 "primary" colors. In addition, changes from one dark color to the next can usually accomplished in relatively short distances minimizing the amount of foam that must be scrapped as a result of the color change. Light shades have proven to be more of a challenge since the color throughput is substantially lower causing the response time to increase before changes actually made in the system can take effect. A means was needed to reduce this response time to an acceptable level thus minimizing the length of time required to change from one color to the next even at flow rates (approaching 2 grams per minute or less.) To do this it was necessary to design a unique 3-way valve/injector system that minimized the volume between the injection port and the recirculation line. This results in a rapid build up of pressure and hence almost instantaneous feed when switching from recirculation to dispense mode. In addition to rapid initiation of color flow it also required that flow be almost instantaneously interrupted even at high throughput when the color was switched from dispensing mode back to the recirculation mode. This is to prevent the "bleeding" of color back into the manifold when the need for color ends. The near immediate start and stop of color flow has been accomplished as a result of the current invention.
Polymeric colorants (i.e., polyoxyalkylenated colorants) such as those described in U.S. Pat. No. 4,284,279 to Cross et al., herein entirely incorporated by reference, have been used for a number of years to color polyurethane slabstock foam (i.e., in a continuous process). Prior to the utilization of such polymeric colorants, pigment dispersion were the main source of polyurethane coloring compounds. Such dispersions have traditionally proven very difficult to handle, too viscous for use within standard injectors, highly staining and thus difficult to clean from standard injector equipment (without the need for environmentally unfriendly solvents), and very abrasive and thus potentially damaging to the delicate machinery associated with coloring slabstock polyurethane foam. As a result, polymeric colorants are widely accepted as the best materials for coloring polyurethane foam.
In the past, custom blends of polymeric colorants were made ahead of time using two or more "primary" colors prior to incorporation within the target foam. The components would be mixed together using some typed of agitation such as mixer or drum tumbler. Once the blend was of an appropriate shade it was transferred to a storage tank for further introduction within the foam substrate. Upon completion of coloring with a specific batch of polymeric colorant, the previously run color would have to be emptied from the storage tank; the tank would need to be cleaned; and then the next color to be run in the same tank would have to be charged in the tank. Cleaning of the tanks, pipelines, etc., was facilitated due to the water-solubility of the polymeric colorants (particularly as compared to pigments); however, the procedures followed were still considered labor intensive and not cost efficient. The general practice was then modified to maintain a dedicated tank for each separate color (shade) that was to run. This led to a number of inefficiencies and limitations that were not desirable if a foam manufacturer was to adequately meet demands in the market place.
Polymeric colorants such as those cited above in Cross et al. were designed to be totally miscible with one another as well as with most polyols, one of the two main ingredients used to produce polyurethane materials (isocyanates being the other). Pigment dispersions, on the other hand, are particulates dispersed in some type of liquid carrier. They require a high degree of agitation before they satisfactorily blend together to provide a uniform color. As a result the short amount of time that the polyol and colorant are mixed in the typical slabstock foam machine's mixhead is not sufficient to produce in an adequate mixture of components to insure a single, homogeneous coloration throughout the target foam. Thus, another modification was made permitting separate addition of desired polymeric colorants within a polyol manifold for subsequent blending as the polyol/isocyanate mixture passes through the mixhead. As a result, well over half of all the colored slabstock foam is produced in the United States through such a method.
A configuration of this new (now typical) polymeric colorant production line for slabstock foam is depicted in FIG. 1. This new coloring system itself generally consisted of 4 to 6 "primary" color storage tanks (one of which is depicted as 10 in FIG. 1) each feeding color to at least one positive displacement spur gear pump 12 coupled to a variable speed motor/drive 14 (such as available from Viking). The motor/pump combination 12, 14 was typically run continuously in either recirculation or dispense mode (depending on the position of a 3-way valve 16) to minimize the time required to spool up the motor 14 to the proper rpm and to ultimately achieve the pressure required to initiate color flow into a pre-mix manifold 18 through an injector 20. The throughput pressure was typically measured through the utilization of a pressure gauge 25 attached to the feed line 13 from the pump 12 to the 3-way valve 16. The typical 3-way valve 16 was air actuated and used to direct the flow of colorant from the recirculation feed line 22 to the dispense feed line 24 (to the injector 20) when color flow to the manifold 18 was required. From the manifold 18, the colorant(s) was moved to the mixing head 26 and then further on to color the target slabstock foam (not illustrated). Although this configuration has proven effective in the past, there remain a number of problems associated with this procedure which have heretofore been unresolved.
For instance, the market place demands that a foam producer be able to provide dark shades as well as light shades with a variety of hues and polyol flow rates. Since color is metered volumetrically a wide range of color flow rates are required to insure low enough flow for a minor component in a light shade. In addition, the polyol flow rates can be as low as 10 kg/min and as high as 300 kg/min [color loading is generally stated in parts per hundred polyol (php)]. As the rate at which the polyol flows is reduced so must the color rate be reduced to maintain the same php. For most foams manufactured in the United States the color delivery systems must be able to provide color flow as low a 2 grams/min and as high as 7000 grams/min or more. The rate at which color begins to flow when pumping 5000 grams/minute is generally very different than pumping 5 grams/min until the present invention is incorporated. Prior to this point the general approach was to use a smaller diameter line for the low flow range. Unfortunately, there are distinct limitations on such a small diameter (small bore) feed line, most notably the resultant throughput pressure drop from pumping material several feet through a small diameter line.
Furthermore, the typical polyurethane slabstock foam coloring system has a three-way air actuated ball valve (28 in FIG. 2) that is positioned up near the polyol manifold. Due to the configuration of the available ball valves they are generally located approximately 1 meter from the manifold. As provided by the representation of a standard three-way ball valve assembly in FIG. 2, material metered by the pump enters the top of the three-way ball valve 27 from the storage tank feed line 29 and exits either through the recirculation side 30 or the dispense side 32 depending on how the ball is oriented. FIG. 2 depicts the ball valve 27 when it is oriented in the recirculation mode. Once it is desired to change from recirculation to dispense and back to dispense the ball valve 28 must typically rotate 180.degree. from one side of the valve to the other (although there are some apparati which utilize a 90.degree. ball valve rotation) through the movement of an actuator (not illustrated) attached to an actuator pin 34 which, in turn, fits into an identation (not illustrated) within the ball valve 27. Furthermore, the typical ball valve 28 comprises a single channel 31 to accommodate the flow of colorant to either the recirculation side 30 or the dispense side 32. This single channel is configured at a right angle and thus may contribute to laminar flow problems by requiring the colorant liquid to radically change direction, thereby altering the pressure over the total liquid mass (and thus producing non-uniformity of pressures over the entire liquid colorant).
In addition to this 3-way valve, a device must be used to inject color away from the wall of the manifold to insure adequate subsequent mixing (i.e., to reduce the problems associated with laminar flow through a feed line having a larger diameter than the 3-way valve. Ideally, such a device should function as a check valve to maintain pressure in the line and to stop color flow when switching from dispense to recirculation. Such devices must maintain pressure after the dispensing unit is returned to recirculation mode otherwise the pressure drops below the "cracking" pressure of the check valve spring which will result in even longer startups which, in turn, may translate in to cost overruns or potentially greater amount of off-quality colored foam. Additionally, the resultant pressure drop must be acceptable across a broad delivery range for such injectors to alleviate any other related pressure difference problems.
An entire colorant pumping system (such as discussed with regard to FIG. 1, above) was developed to evaluate a variety of injection systems that closely resembles an actual production unit. It consisted of a spur gear pump from Viking coupled with a full flux vector motor and drive from Baldor. Stainless steel tubing having an outside diameter of 1/4 inch was connected to the discharge side of the pump. The distance from the pump to the 3-way valve was approximately 40 feet. The distance from the standard 3-way valve to the check valve was 3 feet. The motor/pump was run to insure pressure up through the 3 way valve and then it was allowed to dispense to insure that fluid filled the line under pressure from the valve to the check valve. Measurements were then taken of the time required from the moment the 3-way valve is switched from recirculation to dispense and the time that a liquid polymeric colorant actually began to flow at various throughputs. Colorant response time (the time required for colorant to begin to flow from the three-way valve to the injector) was compared with throughput flow rate for this well known system. The results are tabulated below:
TABLE Colorant Response Time (seconds) Flow Rate (g/min) 48 2.5 15 4 5 20 3 42 0 86
Thus, at low throughput flow rate, the time before delivery becomes excessive. It initially took 48 seconds from the time the valve was rotated until color began to flow at 2.5 grams per minute. This would represent almost 14 feet of off-quality foam generated with the conveyor speed of 17 feet per minute or a loss of up to 700 lbs of foam making chemical that would be disposed of as scrap. Obviously, an instantaneous delivery was needed for all flow rates which has not been accorded the industry by the prior art.